It is well known that a helmet normally consists of a shell, which is the rigid external structure that can be seen from the outside, and is responsible for providing rigidity to the helmet and absorbing the first impact in case of a fall or collision and abrasion with the contact surface. Shells can be manufactured using thermoplastic materials (in helmets with a simpler design) and fibre-reinforced composite materials such as, for example, fibreglass, carbon fibre and Kevlar®, etc., to better absorb blows and also achieve a good resistance-lightness ratio.
The interior of the helmet, disposed on the inner surface of the shell, is a very important part as it is responsible for absorbing the impact in case of accident, due to which it must adapt to the helmet user's head in the best possible way. To this end, a filling made from impact-absorbing material is usually disposed between the shell and the internal liner of the helmet, such as pads or polystyrene foam elements. The configuration of the internal elements must adapt to the mould of the impact-absorbing material and to the anatomy of the zone of the head where they are disposed, and may have different densities according to the zone.
Lastly, all the helmets have an internal liner which, depending on the model, may be detachable in order to be washed independently from the helmet. The liner material is usually breathable in order to evacuate the sweat generated in the interior of the helmet. Mention should also be made of the cheek pads, which in some helmets are also detachable and may come in different sizes and thicknesses in order to adapt to the users.
Furthermore, it is also known that not everyone has the same head size and that there are several head types that can be classified, by shape, into round, flat (or globe), oval (taller-than-wide), egg or inverted egg. In fact, it has also been observed that persons of the same race have certain features in common as regards their head shape depending on the ethnographic group to which they belong. Thus, for example, it has been observed that Caucasian users tend to have different head shapes than Asian users. Nonetheless, there are differences in size (sizes XL, L, M, S, XS) and shape even within the same ethnographic group.
Despite the fact that this variation in terms of head size and shape is known, helmet brand manufacturers do not always offer shells adapted to each user but rather, for example, in the best of cases, manufacture models grouped by country; for example, a single helmet for Europe, Australia and South America, another helmet for the United States, Mexico and Canada, and a third helmet for Asia. Within said geographic areas, some manufacturers offer different-sized shells (XL, L, M, S, XS), with the ensuing cost, while others, in an attempt to palliate the lack of helmet sizes or shapes, manufacture either two or more different-sized shells or a single shell but combined with two or more types of internal foam elements. Also, in general, each brand manufacturer has its own shell mould style and if the user has a head shape that does not match the mould, he or she will have to choose a different brand or accept a less than perfect ergonomic fit.
In addition to the foregoing, it should also be taken into account that, in winter, motorcyclists usually place their helmet over a balaclava, as opposed to summer, due to which the user may be uncomfortable in winter if he or she bought a helmet that fits tightly over his or her bare head. The opposite case would also be unfavourable, because if he or she bought the helmet trying it on over a balaclava, in summer the helmet will not fit tightly.
Therefore, this highlights the problem arising from the lack of adaptation of helmets to user head sizes and morphologies, causing discomfort to the user as a result of the lack of ergonomic fit and thereby negatively affecting active safety.
Recently, protective liners have been developed that are intended to be placed in the interior of the helmet, formed by one or more cells or chambers that may be inflated by pressurised air, interconnected or not by means of channels. These protective liners are inflated by the user by actuating a small inflating pump provided in the helmet, for example by pressing a button, and the level of inflation can be regulated by a valve also provided in the helmet. Protective liners have a shape adapted to cover one or more zones of the head and their level of inflation will cause the space between the helmet and the user's head to be occupied by the inflated liner. Protective liners such as those described were previously developed by the applicant and/or inventors themselves, being one of the products currently manufactured and marketed. Other examples are those described in patent documents FR2888728-A1, FR2918849-A1, U.S. Pat. No. 6,817,039-B1 and U.S. Pat. No. 8,544,117-B2.
Inflatable liners should enable helmet brand manufacturers to offer less product references in terms of sizes, saving on manufacturing and distribution costs and, at the same time, provide any user, regardless of the shape of his or her head, with optimum comfort and safety using any helmet.
Despite the improvement represented by some of these liners, it should be taken into account that the user should not only be protected but also comfortable with the inflatable liner. In addition, some liners, on becoming inflated, do not achieve optimum inflation and their original shape is distorted under operational conditions of use, thus losing efficiency.
At the same time, these liners should also facilitate the perspiration of the helmet user's head and be considerably more durable than conventional foam liners, which become deformed over time, losing volume and leaving a much larger empty space in the interior of the helmet, as though it were a size larger than the original.
In addition to the user's comfort and the reduction in the number of manufacturer references, another aspect to be improved in current helmets is the level of passive safety.
In fact, it is well known that the basic function traditionally assigned to a helmet is the limitation of maximum surface pressure generated by an impact on the skull by means of the distribution of radial forces through the shell over a larger area and the absorption of energy of said impact through the controlled deformation of the shell and of the impact-absorbing material, all in a radial direction. “Radial direction” of impact is understood to be all those impacts which, initiating from the exterior of the helmet, are concurrent in the middle of the head. In current practice, all certification standards and test methodologies applied use said radial impact typology.
In the last two decades, in the field of biomechanical research in the area of accidentology, it has become evident that:                a) In a large number of accidents (motorcycle, but also bicycle, ski, horseback riding and in most sports in which helmets are normally used), the direction of impact of the helmet is not purely perpendicular with respect to the contact surface (which would generate purely radial impacts such as those described previously and applied in most standards), but rather said impacts are basically oblique (the direction of impact with respect to the surface occurs at an angle α where 90°>α>0° and, preferably, 60°>α>15°), therefore involving contact forces with both a radial and tangential component.        b) It has been observed that said contact force with a tangential component is particularly relevant in the generation of all the most common modes and types of head accident injuries. Therefore, said tangential force component, scaled up by the inertia of the head, generates rotational accelerations in the head of very brief pulse and duration but with a high level of intensity. When the brain tissue, but also the brain/cerebrospinal fluid/cranium as a whole, is subjected to said field of accelerations, distribution of stress and tension is generated (predominantly of the shearing type, as understood in mechanical engineering) which can cause most of the injuries commonly described in scientific literature on cranial accidents if certain limits are exceeded.        c) Therefore, it is currently more accepted by the scientific community that both radial and tangential components in the direction of impact are present in nearly all accidents and that both contribute to the probability and severity of the hypothetical injury as a consequence of the linear and rotational accelerations generated respectively. Furthermore, it is recognised that, while current helmets significantly attenuate linear accelerations, their contribution to the reduction of rotational accelerations is minimal, if not non-existent.        
In reference to the research carried out in the field of biomechanics in the area of accidentology, the papers published by Dr. Peter Halldin are cited herein (http://www.researchgate.net/profile/Peter_Halldin/publications).
To date, these advances in accident research have generated different product solutions or implementations aimed at limiting said rotational accelerations. Examples of these solutions are those described in patent documents U.S. Pat. No. 8,578,520-B2, EP2523572-A1, EP2114180-B1 and EP1404189-B1.
U.S. Pat. No. 8,578,520-B2 discloses a helmet comprising an energy-absorbing layer and an attachment device for securing the helmet to a user's head, wherein a sliding facilitator is provided inside the energy-absorbing layer, said facilitator being fixed to the attachment device and/or to the interior of the energy-absorbing layer and the attachment device to provide sliding between the energy-absorbing layer and the attachment device. The helmet also comprises a casing or shell disposed outside of the energy-absorbing layer. The sliding facilitator is a low-friction material connected to or integrated with the attachment device on the surface oriented towards the energy-absorbing layer and/or disposed on or integrated in the inner surface of the energy-absorbing layer oriented towards the attachment device.
Patent application EP2523572-A1 discloses an intermediate layer of a friction-decreasing material disposed between two layers. This intermediate layer is adapted to create a sliding movement between two layers when a force is applied and a tangential force component shears the layers. The friction-decreasing material comprises fibres, all or some of which may be natural fibres and/or polymer fibres.
Patent EP2114180-B1 makes reference to a locking device for fixing the position of an outer layer with respect to an inner layer in a protective helmet, wherein the protective helmet has a sliding layer disposed between the outer layer and the inner layer to facilitate the movement of the outer layer with respect to the inner layer during an oblique impact towards the protective helmet. The locking device comprises a guiding member of the layer, which has an upper portion intended to be disposed at an opening of the outer layer and a resilient lower portion extending from the upper portion and which, at its free end, is disposed in connection to the inner layer.
Patent EP1404189-B1 discloses a protective headgear comprising a shell having an inwardly facing surface which in use faces the head of a user and an outwardly facing surface which in use faces away from the head of a user. An outer layer overlies a portion of the outwardly facing surface of the outwardly facing shell and rupturing means are provided for fixedly attaching the outer layer to the remainder of the headgear at one or more locations. The rupturing means are configured so as to fail when a force greater than a selected threshold is received on an outer surface of the headgear which acts in at least part tangential direction to rotate the headgear and the head of the user. Upon failure of the rupturing means at the one or more locations, the received force causes at least part of the outer layer receiving the force to move relative to the shell in a manner which is similar to the protective movement of the human scalp relative to the skull.
Despite the enhancements achieved in the aforementioned helmets, the need to provide an alternative capable of minimising or reducing the rotational acceleration suffered by a user's head in case of accident, thereby reducing the risk and severity of the injuries that does not imply adding or considerably modifying the helmet's components, is evident.